For decades, the V8 has represented power in the automotive world. Eight cylinders, a compact block, a deep exhaust note, and enormous torque made the configuration almost inseparable from American trucks.
Yet something different happened when diesel engines moved into the hardest-working applications.
Heavy trucks, industrial equipment, long-haul tractors, and some of the most respected diesel pickups increasingly became associated with one layout: the inline-six.
That does not mean every V8 diesel disappeared. Ford still offers its 6.7-liter Power Stroke V8 in the 2026 Super Duty, producing as much as 500 horsepower and 1,200 lb-ft of torque, while Chevrolet continues offering its 6.6-liter Duramax V8 in the Silverado HD.
But when the requirement becomes sustained heavy-duty work rather than simply maximum pickup-truck output, the straight-six repeatedly appears.
Why?
Because diesel engines reward exactly the characteristics the inline-six naturally provides: balance, low-speed torque, structural simplicity, easy servicing, and the ability to scale to enormous displacement.
The Inline-Six Is Naturally Smooth
The first major advantage is mechanical balance.
An inline-six has six cylinders arranged in a single row, with crankshaft geometry that allows primary and secondary forces to balance naturally. That helps the engine operate smoothly without requiring the same level of additional balancing hardware that some other configurations may need.
That matters particularly in diesel engines.
Diesel combustion produces extremely high cylinder pressures. Those forces travel through pistons, connecting rods, bearings, the crankshaft, and ultimately the engine block.
A layout that manages vibration effectively creates a strong foundation for an engine expected to operate under substantial load for thousands of hours.
The importance of this basic architecture can still be seen in modern heavy trucks. Volvo’s current D13 is a 13-liter inline-six diesel designed specifically around what Volvo describes as a balance of efficiency, performance, and reliability.
At the extreme end of the market, Volvo’s current 17-liter inline-six produces as much as 780 horsepower and 3,800 Nm of torque for demanding heavy transport.
That is an important clue.
The inline-six is not surviving because manufacturers are unwilling to change. It continues appearing because its fundamental geometry works exceptionally well for large diesel engines.
Diesel Engines Care More About Torque Than Cylinder Count
The V8 sounds impressive because eight cylinders suggest more power.
But cylinder count alone does not determine whether an engine is suitable for serious diesel work.
Heavy vehicles need torque at relatively low engine speeds.
A loaded tractor climbing a hill does not benefit from an engine that needs extremely high rpm to reach useful output. It needs substantial torque early and needs to maintain that pull continuously.
Large inline-six engines are particularly well suited to this operating philosophy because they can use substantial displacement per cylinder, long strokes, strong crankshafts, and turbocharging to generate enormous low-speed torque.
Cummins’ current 6.7-liter diesel used in Ram Heavy Duty pickups demonstrates the principle. Ram lists the 2026 high-output Cummins inline-six at 430 horsepower and 1,075 lb-ft of torque.
The important figure is not merely horsepower.
It is the enormous torque produced by only six cylinders.
Cummins has continued developing the architecture because modern turbocharging, injection systems, combustion control, and electronics allow six large cylinders to produce levels of output that once would have required substantially different designs. Its latest 6.7-liter platform emphasizes power, efficiency, serviceability, and drivability rather than increasing cylinder count.
Six Cylinders Can Mean Fewer Parts
An inline-six contains only one cylinder bank.
That sounds obvious, but it creates important engineering consequences.
A V8 has two cylinder banks. Depending on its design, that can mean two cylinder heads, two exhaust manifolds, more complicated intake routing, and additional packaging challenges around turbochargers, emissions equipment, cooling systems, and accessories.
The inline-six places every cylinder in one straight line.
That can simplify access to major components and make the engine easier to service, particularly when it is installed longitudinally in a large truck.
For commercial operators, serviceability is not merely convenient.
Downtime costs money.
A truck sitting in a workshop is not hauling freight.
Cummins specifically promotes serviceability as one of the characteristics of its current 6.7-liter pickup diesel, while its larger X15 heavy-duty engines are similarly developed around commercial operating costs and maintenance requirements.
In that environment, a slightly narrower component count can matter much more than whether an engine looks compact when viewed from above.
The V8 Wins an Important Battle: Length
If the inline-six is so effective, why do Ford and Chevrolet continue using V8 diesels?
Packaging provides much of the answer.
An inline-six is long.
All six cylinders sit one behind another, producing an engine that can take considerable longitudinal space.
A V8 divides those cylinders into two shorter banks. The resulting engine can be considerably shorter, even if it is wider.
That shape works well in pickup trucks.
The engine needs to fit between the front axle, firewall, cooling hardware, suspension components, crash structures, and increasingly complicated emissions equipment.
A V8 can therefore provide enormous displacement and power in a shorter package.
That is why Ford’s modern Power Stroke remains highly competitive rather than somehow being technologically defeated. The 2026 high-output 6.7-liter Power Stroke produces 500 horsepower and 1,200 lb-ft of torque.
Chevrolet’s 6.6-liter Duramax likewise produces 470 horsepower and 975 lb-ft in the current Silverado HD.
In pickups, the V8 still makes enormous sense.
In long-haul tractors, however, engine length is often easier to accommodate.
That changes the engineering calculation.
Inline-Sixes Scale Extremely Well
One of the straight-six’s greatest strengths becomes obvious when displacement increases.
Volvo currently builds six-cylinder inline diesels at 13 and 17 liters, while Cummins has spent decades producing large inline-six engines for heavy equipment and commercial applications.
Cummins’ K19, for example, is a 19-liter inline-six architecture that has remained in the company’s product range for decades. Cummins describes it as a platform aimed at low total cost of ownership.
That means each cylinder displaces more than three liters.
Trying to achieve the same displacement through eight cylinders would introduce additional components without automatically producing benefits that a low-rpm industrial diesel actually needs.
Heavy-duty diesels are not chasing 7,000-rpm redlines.
They need huge combustion events, durable components, sustained torque, and predictable operation.
The straight-six handles that assignment extremely well.
The Crankshaft Gets Seven Main Bearings
Large inline-six engines can also support their crankshafts extensively.
A typical heavy-duty straight-six can use seven main bearings, placing a bearing between each neighboring crank throw and at both ends of the crankshaft.
Volvo specifically highlights seven bearings distributing forces in documentation for its inline-six heavy-duty engines.
That matters when cylinder pressure becomes enormous.
Diesel crankshafts experience tremendous loads repeatedly, particularly during towing or long periods at high engine load.
A heavily supported crankshaft combined with relatively low engine speeds provides a strong recipe for longevity.
This does not automatically make every straight-six more reliable than every V8.
Engine design, materials, cooling, lubrication, emissions equipment, maintenance, and operating conditions still determine reliability.
But the architecture starts with characteristics that suit heavy diesel operation remarkably well.
Turbocharging Removed the Need for Extra Cylinders
Another reason six cylinders became enough is turbocharging.
Modern diesels can force large quantities of air into relatively few cylinders. High-pressure common-rail injection then delivers precisely controlled quantities of fuel.
The result is extraordinary torque density.
A six-cylinder engine therefore no longer needs additional cylinders simply to produce competitive output.
The latest Volvo D17 proves how far that concept can be pushed: six cylinders, 17 liters, up to 780 horsepower, and 3,800 Nm of torque.
At that point, adding two more cylinders would not automatically solve a power problem.
There may not be one.
The V8 Never Really Lost—The Inline-Six Simply Found the Perfect Job
The idea that every diesel V8 “lost” makes a great headline, but the engineering reality is more interesting.
The V8 remains extremely successful where compact length and huge pickup-truck output matter. Ford and General Motors continue proving that today.
The inline-six dominates many heavy-duty environments for a different reason.
Its architecture aligns almost perfectly with what large diesel engines need.
It is naturally smooth. It supports large displacement. It can generate enormous low-rpm torque. It keeps the cylinder arrangement mechanically straightforward. Its crankshaft can be heavily supported, and its single-bank configuration can simplify servicing.
That is why an engine layout that looks almost old-fashioned continues powering some of the most advanced trucks being produced.
The inline-six did not win because eight cylinders stopped working.
It won because when the assignment is pulling enormous weight for enormous distances, six cylinders arranged in one straight line are remarkably difficult to improve upon.
